MATERIALS AND METHODS

Sampling location: The study area is located at Kuantan estuary adjacent to Tanjung Lumpur mangrove area (Fig. 1). It is a traditional fishing village located along the mouth of the Kuantan estuary. This study area lies between longitude 103°19’13.09” E to 103°21’02.79” E and latitude 03°48’46.34” N to 03°47’34.82” N.

Sample collection and preparation: A total of 50 Solen brevis (Shell length 5-8 cm) were collected randomly by hand picking during lowest low tide time from January and April 2010 at sandy beaches of Tanjung Lumpur, Kuantan Pahang to examine essential and Non essential heavy metal accumulation in the soft tissue part. Collected mussels were placed in sterile plastic bags and iced prior to laboratory analysis. The samples were brought to the Institute of Oceanography and Maritime studies (INOCEM) lab and cleansed to remove the mud or any attached particles and then washed with double distilled water. Soft tissue (edible portion) from the shells were excised with a plastic knife and wrapped with aluminum foil and dried in oven at 65°C for 72 h.

Tissue digestion procedure: Acid digestion method was performed to digest the samples which involved heating of 0.5 g of dried tissues of razor clam in Teflon beaker with mixed concentrated acids (Hydrogen Peroxide (H₂O₂), Nitric acid (HNO₃), hydrochloric acid (HCl) ) and sulphuric acid (H₂SO₄) in the ratio of 1:1 (Kamaruzzaman et al., 2007). After the digestion process hundred times dilution was performed using Mili-Q water then the samples were analyzed using Inductively Coupled Plasma Spectrophotometer (ICP-MS). The values of the heavy metal concentrations in the tissues were calculated based on dry weights as this discounts the variability due to inner parts differences in the moisture content of organisms. The precision assessed by replicate analyses was within ±3%. The analytical procedures for the razor clam were checked with the Standard Reference Material (SRM) for Oyster Tissue 1566b. The recoveries of all the metals were between 95-105% (Table 1). One-way Analysis of Variance (ANOVA) statistical test was performed to check the significance of p-value. The determined concentration of various heavy metals were compared with previous study on the heavy metal accumulation in the soft tissues of Solen regularis collected from Sabah, Malaysia (Kanakaraju et al., 2008b).

Fig. 1:	Location of the study area showing sampling site at Tanjung Lumpur Coastal area, Pahang

Table 1:	Percentage recovery test for Standard Reference Material (SRM) oyster tissue 1566b

Table 2:	Comparison of analyzed heavy metal concentrations (μg g-1 dry weight) in the soft tissue of Razor clam (Solen brevis) in the present study with similar studies on Solen regularis and the permissible limits set by Food and Agriculture Organization/World Health Organization (FAO/WHO) and Malaysian Food Regulation (MFR, 1985)

RESULTS

The concentration of analyzed metals (Fe, Zn, Mn, Cu, Pb and Cd) in the soft tissues of Solen brevis revealed that, Iron (Fe) concentration in the edible soft tissues of razor clam collected from study area (Tanjung Lumpur coastal area, Kuantan) was higher than the other metals with the mean concentration of 415.2±56.52 μg g^-1 followed by Zinc (Zn) with mean concentration of 87.74±11.85 μg g^-1. Mean concentration of Manganese (Mn) and Copper (Cu) in the soft tissue was 18.71±2.10 and 8.64±1.75 μg g^-1, respectively. Lead (Pb) and Cadmium (Cd) were accumulated in lower concentration in the soft edible tissue of Solen brevis with the mean concentration range of 1.61±0.45 and 0.67±0.29 μg g^-1, respectively. Present observation also revealed that the bioaccumulation level of essential metals is greater than the non-essential metals in the soft tissue of the organism. The order of metal accumulation in the soft tissue of S. brevis collected from Tanjung Lumpur coastal area, Kuantan was Fe > Zn > Mn > Cu > Pb > Cd in this present study (Table 2).

It was also observed that Fe, Cd accumulation in the soft tissue of razor clam collected from Tanjung Lumpur coastal area was lower than the razor clam samples from Serpan, Sabah but higher than Moyan, Sabah samples. Mn concentration in the soft tissue of razor clam in the present study was lower than the concentration of Mn from the Sabah samples but Cu concentration was comparatively elevated in the razor clams in the present study when compared with both Moyan and Sepan samples.

DISCUSSION

The knowledge of heavy metal concentrations in native species is very important with respect to nature management, human consumption of these species and to determine the most useful biomonitor species and the most polluted area. So, the present study examined the concentrations of Fe, Zn, Mn, Cu, Pb and Cd in the soft tissue of razor clam collected from Tanjung Lumpur coastal area. It is also to be noted that there are various factors influencing the metals accumulation in bivalves. Among them metal bioavailability, season of sampling, size of the sample, hydrodynamics of the environment and reproductive cycle of selected organism are having direct influences on the metal accumulation in to the different body tissues (Otchere, 2003; Kamaruzzaman et al., 2008b). The elevated concentration of Fe in the soft tissue of razor clam compared to the other metals in the present study might be due to the major role played by this essential metal in maintaining the proper physiological functions of the organism. This observation was well corresponded with the previous study by Kanakaraju et al. (2008a) who postulated that Fe plays an important role as an essential element in all living systems from invertebrates to human, hence they tend to accumulate high concentration of Fe from the surrounding environment. The high concentration of Fe in the razor clam might also be due to the abundance of this element in substrate/ sediment. Furthermore, this occurrence also indices the natural capacity to regulate and accumulate elevated concentration of Fe in bivalves (Kanakaraju et al., 2008b).

Even though the Zn concentration was higher in the soft tissues of razor clam when compared with other metals (except Fe), their accumulation produces no risk to the aquatic organisms.

Fig. 2:	Linear regression analysis shows relationship between shell length (cm) and the amount of accumulation in the soft tissue of razor clam (μg g-1 dw) collected from Tanjung Lumpur coastal waters

Similar observation was reported by Kamaruzzaman et al. (2008a) in Asian green mussel Perna viridis indicating higher accumulation of Zn concentration in the soft tissue compared to other toxic metals studied. It was also observed that it plays a major role in the biosynthesis of nucleic acids, DNA and RNA polymerases production and thus involved in the healing processes of tissues in the body through various enzymatic reactions (Bodsworth, 1994).

The observed low concentration of Mn in the soft tissue of razor clam collected from Tanjung Lumpur coastal area compared with the earlier studies clearly showed that the sources of Mn in the study area was relatively limited. On the other hand, Cu accumulation in the samples from the present study was comparatively higher than the samples from other sampling locations indicating the source of Cu in Tanjung Lumpur coastal area was relatively higher than the other sampling sites. This observation might also be due to the physiologic condition of the razor clams that is influenced by the existing environmental factors which in-turn accelerates the Cu accumulation in faster rate.

In contrast, Cd is not an essential element for plants, animals and human beings (Kamaruzzaman et al., 2010; Merian, 1990). Generally, the occurrence of Cd in the marine environment is rare. Therefore, the impact of Cd on the environment is considerably small. As a result, this study shows Cd as the least abundant metal in razor clam. It was reported that normally bivalves do not regulate Cd, but they usually accumulate this element (Kamaruzzaman et al., 2008a; Li et al., 2006). Hence, razor clam might not be able to regulate Cd in their body. It was also observed that Pb accumulation in the soft tissue was also under permissible limit indicating the Tanjung Lumpur coastal area is not polluted by lethal heavy metals such as Cd and Pb. Linear regression analysis showed the shell length had no influence on the amount of heavy metal accumulation by the razor clam for all the selected metals (except for Zn) and it was also observed that razor clam with the shell length range between 6.5-7.5 cm accumulates more amount of heavy metals (Fig. 2).

Metals concentrations obtained in this study were compared with the international standards for metals in mollusks/shellfish compiled by Food and Agricultural Organization (FAO) of United Nation and Malaysian Food Regulation as shown in Table 2. The obtained results showed that, all metals were accumulated in lower concentration in the soft tissue than their respective Maximum Permissible Limits (MPL). In comparison with previous study done by Kanakaraju et al. (2008a), present study confirm relatively higher concentration of Cu and Zn, whereas, Mn, Fe, Cd and Pb occur in slightly lower concentrations in the soft tissue of Solen brevis.

CONCLUSION

Present study revealed that the concentration of heavy metals such as Fe, Zn and Mn in the soft tissue of razor clam collected from Tanjung Lumpur coastal waters found to be high in the samples from other sampling locations and the least abundant element was Cd (Kanakaraju et al., 2008b). The detected concentration of all the studied metals in the soft tissue was fall within the regulatory limits set by FAO and Malaysian Government. Thus, it can be concluded that the razor clam collected from Tanjung Lumpur can be classified as safe for human consumption. In addition, by using Solen brevis as a biomonitoring agent, the contamination of Mn, Fe, Cu, Zn, Cd and Pb in this study area was found not be serious since Solen sp. accumulates heavy metals in the soft tissues and constitutes one of the important food-chains in the coastal environment, this information is therefore useful in predicting metal contaminations in the coastal areas.